The present invention relates to radio frequency receivers, and more particularly to a method of conversion spur avoidance in a multi-conversion radio frequency receiver.
A multi-conversion radio frequency receiver may use multiple local oscillators when up-converting and down-converting a signal of interest. These separate local oscillators create sum and difference harmonic mixing products between themselves using the various nonlinear elements, primarily mixers and amplifiers, in the receiver. The amplitude of these harmonic mixing products depends on the conversion efficiency of the various elements at the harmonic mixing product frequencies. As a first local oscillator (LO1) is tuned to convert assorted radio frequency (RF) input frequencies to a first intermediate frequency (IF1), the unintended harmonic mixing products of the LO1 frequency with a second local oscillator (LO2) frequency may fall within an IF1 filter bandwidth. When this occurs in a spectrum analyzer instrument, the energy from the unwanted harmonic mixing product, or conversion spur, appears as a real RF signal on a final spectral display.
What is desired is a method of conversion spur avoidance that predicts and avoids unwanted harmonic mixing products from multiple local oscillators in a multi-conversion radio frequency receiver.
Accordingly the present invention provides a method of conversion spur avoidance in a multi-conversion radio frequency receiver by predicting where conversion spurs may occur and retuning a second one of the local oscillators when the predicted conversion spur falls within an IF filter bandwidth of a first one of the local oscillators. During design of the radio frequency receiver a table of problem frequencies for the first local oscillator relative to a frequency of the second local oscillator which produce harmonic mixing products that fall within the bandwidth ofthe IF filter having a given center frequency IF1 is generated. For each problem frequency and a frequency offset is determined for the second local oscillator which moves the harmonic mixing product out of the IF filter bandwidth. Where the center frequency IF1 of the IF1 filter varies from receiver to receiver, entries into the table are made for each possible IF1 frequency and during calibration of the receiver the IF1 frequency is precisely measured to determine the entries of the table to use during operation. In operation when one of the problem frequencies occurs, the second local oscillator is retuned by the corresponding frequency offset from the table so the harmonic mixing product is moved out of the IF1 filter bandwidth. The direction of the frequency offset for the second local oscillator may be determined by which half of the IF filter bandwidth the harmonic mixing product falls.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.